1. Field of the Invention
The invention is directed to resistivity anisotropy interpretation systems and methods for well logging application and, in one particular aspect, to a data interpretation system and method that is usable to determine formation parameters and reservoir descriptions in real-time.
2. Description of the Related Art
The production of hydrocarbons from subsurface formations typically commences by forming a borehole through the earth to a subsurface reservoir thought to contain hydrocarbons. From the borehole, various physical, chemical, and mechanical properties are “logged” for the purpose of determining the nature and characteristics, including for example, the porosity, permeability, saturation, and depth of the subsurface formations encountered. One such logging technique commonly used in the industry is referred to as induction logging. Induction logging measures the conductivity or its inverse, the resistivity, of a formation. Formation conductivity is one possible indicator of the presence or absence of a significant accumulation of hydrocarbons, because, generally speaking, hydrocarbons are relatively poor conductors of electricity. On the other hand, formation water, which is typically salty, is a relatively good conductor of electricity. Thus, induction logging tools can obtain information that, properly interpreted, indicates the presence or absence of hydrocarbons.
These induction (also known as electromagnetic induction) well logging instruments were first introduced by Doll, H. G., “Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Based Mud”, Journal of Petroleum Technology, vol. 1, pp.148-62, Society of Petroleum Engineers, Richardson Tex. (1949). Induction well logging instruments typically include a sonde having one or more transmitter coils and one or more receiver coils at axially spaced apart locations. Induction well logging instruments also typically include a source of alternating current (AC) which is conducted through the transmitter coils. The AC passing through the transmitter coils induces a magnetic field within the surrounding formations, causing a flow of eddy currents within the earth formations. In general, the magnitude of the eddy currents is proportional to the electrical conductivity (the inverse of the electrical resistivity) of the earth formations surrounding the instrument. The eddy currents, in turn, induce a magnetic field that is coupled to the one or more receiver coils, thereby inducing in the receiver coil(s) a voltage signal with magnitude and phase dependent upon the electrical characteristics of the adjacent formation.
Induction logging technology has evolved significantly since its introduction by Doll. In recent years, induction devices consisting of several complex coil combinations have been replaced by tools with multiple arrays. See, for example, Beard, D. R. et al., “Practical Applications of a New Multichannel and Fully Digital Spectrum Induction System, 1996 SPE Annual Technical Conference and Exhibition, Denver, Colo., SPE-36504, Oct. 6-9, 1996, pp. 99-109, which is referred to, for example, in U.S. Pat. No. 6,219,619 issued to Xiao et al. Each array consists of one transmitter and a pair of receiver coils. These new induction devices are commonly referred to as array-type induction tools.
The older style induction tools attempt to focus the tool response using carefully selected coil arrangements. The focusing therefore is fixed by the tool design, i.e. these tools are “hardware-focused”. A hardware focusing method have been proposed by Moran and Chemali (see, for example, J. H. Moran and R. Chemali, 1985, “Focused resistivity logs”, in Developments in Geophysical Exploration Methods, v.6, A. A. Fitch, ed., Applied Science Publishers, Ltd., London, p. 225-260). In the new array-type induction tools, the measurements from various arrays are combined through a software algorithm to achieve focusing of the signal response, i.e. these tools are “software-focused”. This processing produces a set of curves with predetermined depth of investigation, and vertical resolution.
Using software-based focusing provides greater flexibility for handling various logging environments and for creating more reliable induction logs. However, the quality and accuracy of the final focused logs is dependent on the accuracy of the software focusing method. Current software focusing methods have been proposed by Barber (see T. D. Barber et al., “Using a Multiarray Induction Tool To Achieve High-Resolution Logs With Minimum Environmental Effects”, 66th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Dallas, Texas, SPE-22725, Oct. 6-9, 1991, pp. 637-651.) and by Zhou (see Zhou et al., “Numerical Focusing of Induction Logging Measurements”, 12th Workshop in Electromagnetic Induction in Earth, International Union Geodesy and Geophysics, Aug. 8-14, 1994, Brest, France, p. 10.), are referred to, for example, in U.S. Pat. No. 6,219,619 issued to Xiao et al. The software focusing methods provide reliable estimates of the true formation resistivity, Rt (Rh).
U.S. Pat. No. 5,452,761 to Beard et al., the contents of which are fully incorporated herein by reference, discloses an apparatus and method for digitally processing signals received by an induction logging tool comprising a transmitter and a plurality of receivers. The received voltages are digitized at a sampling rate well above the maximum frequency of interest. The digitizing window is synchronized to a cycle of the oscillating current signal. Corresponding samples obtained in each cycle are cumulatively summed over a large number of such cycles. The summed samples form a stacked signal. Stacked signals generated for corresponding receiver coils are transmitted to a computer for spectral analysis. Transmitting the stacked signals instead of all the individually sampled signals reduces the amount of data that needs to be stored or transmitted. A Fourier analysis is performed on the stacked signals to derive the amplitudes of in-phase and quadrature components of the receiver voltages at the frequencies of interest. From the component amplitudes, the conductivity of the formation can be accurately derived.
The effect of formation anisotropy on resistivity logging measurements have long been recognized. Kunz and Moran studied the anisotropic effect on the response of a conventional logging device in a borehole perpendicular to the bedding plane of a thick anisotropic bed. U.S. Pat. No. 6,219,619, issued to Xiao et al., discloses a method of software focusing for array-type induction logging tools using an inhomogeneous background formation model in a vertical well. Using this inhomogeneous background formation model, the formation response of the induction logging tool can be split into two portions: a background response, and a certain “response residue”. The background response is obtained as computer simulated measurements of the inhomogeneous background model. The response residue is the difference between raw measurements and the background responses. Xiao '619 reduces nonlinearity effects and thereby improves the focusing method. The method of Xiao '619 requires a significant amount of processing time for forward modeling, data inversion, skin effect corrections, etc. This limits their effectiveness as a method for real-time analysis.
Another technique used in oil exploration and well logging is a lateral log. Lateral logging techniques are taught in Doll, H. G., “The Laterolog”, Paper 3198, in Transactions of the AIME, v 192, p. 305-316, 1951, and in Doll, H. G., “The Microlaterolog”, Paper 3492, in Transactions of the AIME, v 198, p. 17-32. Generally, the laterolog is an electrode device with multiple current electrodes configured in several different ways to produce several different responses. A current-emitting and current-return electrodes (A and B) are placed close together on the sonde, with a measure electrode (M) several feet away, and a measure return (N) far away. This arrangement is sensitive to the potential gradient between A and B. The Array Lateral Log technology of data measurements and interpretation is taught in Hakvoort et. al paper “Field Measurements and Inversion Results of the High-Definition Lateral Log”, Paper C, in Transactions of the SPWLA, 1998.
U.S. Pat. No. 6,060,885, issued to Tabarovsky et al., discloses a differential array instrument and a method for determining selected parameters of an earth formation surrounding a borehole. The invention includes an instrument mandrel carrying a single source electrode for injecting an electrical current of a predetermined value into the formation surrounding the borehole, and an array of measurement electrodes uniformly and vertically spaced from the source electrode along the instrument mandrel. The uniformly and vertically spaced electrodes are adapted to derive first and second difference potentials between electrodes. The first and second difference potentials are derived in response to current from the source electrode traveling generally vertically in an orientation generally parallel to the axis of the borehole in the formation to successive ones of the predetermined groups of selected measuring electrodes. The plurality of first and second difference potentials may be correlated to a plurality of values representative of the selected formation parameters. The plurality of values representative of the selected formation parameters may provide a profile of the selected parameters over an increasing radial distance from the borehole. The lateral log is generally not designed for differentiating horizontal and vertical resistivities in a formation having resistivity anisotropy.
Typically, measured data needs to be corrected for effects of the borehole and of invasion. U.S. Pat. No. 6,381,542, issued to Zhang et al., the contents of which are incorporated herein by reference, discloses a method for real-time borehole correction of resistivity logging data. In the first stage, the entire range of possibilities of earth models relevant to borehole compensation is sampled and a suit of tool responses is generated, with and without the borehole. A wide range tool response including the borehole effects is input to a neural net and the neural net is trained to produce the corresponding borehole-free response. Once the neural net has been trained, in the second stage, the neural net is validated by using as input tool responses that were not used in the training of the neural net and comparing the output of the neural net to the corresponding borehole-free response. If the agreement is good, then the neural net has been validated and may be used to process subsequently acquired data that includes borehole effects. The borehole corrected measurements may be inverted using an additional neural net.
Typical resistivity methods use an inversion of the obtained data. Some improved methods for data inversion, including data obtained in anisotropic rock formations, are described in U.S. Pat. No. 5,889,729, issued to Frenkel et al.; in Hagiwara T. and Zea H., 1999, “Identifying and quantifying resistivity anisotropy in vertical boreholes”, 40th Annual Logging Symposium, paper Z.; and in Griffiths R., Barber T., and Faivre O., 2000, “Optimal evaluation of formation resistivities using array induction and array laterolog tools”, 41st Annual Logging Symposium, paper BBB. Frenkel et al. teaches a method for rapid, well-site inversion of resistivity logs. The paper by Hagiwara et al. teaches a method of identifying and estimating resistivity anisotropy in vertical holes. The method is derived from 2D modeling of electric- and induction-log responses. Resistivity anisotropy can be estimated preferably from the difference between these electric- and induction-log resistivity measurements. The paper of Griffiths et al. uses an improved information content of the array measurements, better defines borehole effects, and thus either flags or more accurately corrects data. The authors of Hagiwara et al. and Griffiths et al. suggest using the joint interpretation of focused, Dual Laterolog-type (DLL-type), galvanic logs and conventional array induction-type logs. However, since focused galvanic measurements are not very sensitive to changes of the formation resistivity in the vertical direction (Rv), the techniques offered in Hagiwara et al. and in Griffiths et al. cannot provide reliable anisotropy estimates.
Further methods of determining resistivity are outlined in Yang, 2001, “Determining resistivity anisotropy by joint lateral and induction logs”, SPWLA 42nd Annual Logging Symposium, paper CC. Yang suggests a joint 2-D inversion of lateral and induction logs, however, this inversion process is time consuming and is not applicable for real-time conditions. Rosato and Beck (see Rosato V. and Beck J., 1997, “Real-time interpretation of MWD anisotropy in high angle wells, Offshore Gulf of Mexico”, SPWLA 38th Annual Logging Symposium, paper T.) present a method of quick anisotropy determination using previously stored data tables, which include age, deposition environment, and anisotropy ratios at various dip angles. After deciding which well type lithologic boundaries and anisotropy ratios are expected, a pre-drill model can be constructed to assist in real-time interpretation of a high-angle well.
To correctly determine the anisotropy distribution around the borehole, one must apply time-consuming, inversion-type processing, which is only practical at data processing centers having sufficient computer power. There is a need for a real-time method of approximating the resistivity anisotropy at the well-site. The present invention satisfies this need.